Hybrid RANS-LES simulation of transverse fuel injection in a Mach-10 scramjet engine

TL;DR

This work demonstrates that an IDDES-based hybrid RANS–LES approach, coupled with a 12-species, 27-reaction hydrogen mechanism, can predict auto-ignition locations and unsteady combustion in a full-scale, Mach-10 scramjet with transverse hydrogen injection. By integrating the inlet, injectors, combustor, and nozzle, the study captures hotspot formation, flame stabilization, and multi-regime combustion, validated against schlieren and OH data and analyzed with Takeno flame index and CEMA. Key findings show ignition occurring in partially premixed regions downstream of a shock interaction, with autoignition-driven stabilization supported by explosive modes and Damköhler metrics, aligning well with experimental observations. The framework offers a predictive tool for designing hypersonic air-breathing propulsion systems and highlights the value of high-fidelity, integrated simulations for capturing critical thermo-chemical phenomena at extreme flight conditions.

Abstract

Hypersonic flight poses unique propulsion challenges, requiring engines that maintain thrust, efficiency, and stability across a wide range of operating conditions. These engines must transition smoothly between flight regimes and altitudes. Scramjets (supersonic combustion ramjets) play a key role in addressing these challenges. Recent advancements in high-fidelity computational fluid dynamics (CFD) tools allow researchers to explore novel designs and improve the feasibility of hypersonic travel. In this work, we analyze a radical-farming type scramjet engine mounted at the University of Queensland's T4 Wind Tunnel at Mach 10. We use the Improved Delayed Detached Eddy Simulation (IDDES) model, which combines Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) in different flow regions. A novel integrated modeling strategy is introduced, coupling the inlet, fuel injectors, combustor, and nozzle for full-scale engine analysis. Hydrogen combustion is modeled using a Finite Rate Chemistry (FRC) approach with a 12-species, 27-reaction mechanism to capture shock-induced chemical kinetics across equivalence ratios of $φ= 0.5$ to $0.9$. The Takeno flame index analysis reveals multiple combustion regimes, with ignition occurring in the partially premixed regime. This is supported by Chemical Explosive Mode Analysis (CEMA), which identifies regions of high chemical sensitivity, correlating with observed hot pockets and providing insights into autoignition and flame stabilization mechanisms. The combination of IDDES and FRC improves the transport of hydrogen to hot pockets, producing combustion patterns that match experimental results. This work establishes a framework to address critical challenges in future air-breathing propulsion systems.

Figures (21)

• Figure 1: Schematic of the radical farming type scramjet configuration mounted on the T4 wind tunnel at the University of Queensland Lorrain_aiaa_2012
• Figure 2: Visualization of the structured hexahedral grid highlighting the fuel injector and near-wall region. Top: view of ramp-injector region mesh. Bottom: view of injector mesh cross section.
• Figure 4: A comparison of experimental Schlieren imaging and OH chemiluminescence measurements with numerically generated Schlieren data at the centerplane, along with the OH mass fraction isosurface at 1% for $\phi = 0.6$.
• Figure 5: Cross-sectional planes along the scramjet flow path ($x/L = 0.20 - 1.0$) showing hydrogen dispersion and combustion, with lobes fragmenting downstream of injection and becoming more uniformly mixed through the isolator/combustor. at $\phi = 0.8$.
• Figure 6: Cross-sectional planes along the scramjet flow path ($x/L = 0.20 - 1.0$) of temperature and water vapor distributions for $\phi=0.5$ and $\phi=0.9$, showing higher temperatures and faster fuel consumption at the richer condition.